US Patent publication 20030209221 (the '221 publication, published Nov. 13, 2003) discloses a two-dimensional rotary displacement device comprises a housing, an outer rotor and at least one inner rotor. The axes of rotation of the outer rotor and the at least one inner rotor are parallel. This geometry provides problems such as gyroscopic forces and centrifugal loading of the outer rotor associated with the large spinning mass of the outer rotor.